Generally, fiber obtained by mechanically or chemically treating plant material is referred to as pulp. The pulp material includes cotton, hemp, linen, jute, ramie, Manila hemp, Edgeworthia papyrifera tree fiber, paper mulberry fiber, straw, esparto grass, bamboo fiber, and bagasse, as well as wood. In addition, requirements for industrial material include abundant quantity, easy collection, transportation and storage, low price and excellent quality.
Wood, as a main pulp material, is composed of cellulose, hemicellulose and lignin. These components constitute a cell wall and an intercellular layer, and constitute 90% or more of all trees. Minor components include extracts, such as resin, refined oil, oil fat, tannin and flavonoid, and other inorganic compounds. Among these components, cellulose is present in the largest amount among the natural organic materials, and mainly constitutes the cell wall of the plant. Cellulose is insoluble in water, diluted acid and alkali at room temperature, and is a polymer material having D-glucose subunits linked by β-1:4-glucoside bond. For industrial application, wood cellulose is subjected to processes of beating, bleaching and purifying to manufacture paper, or the wood may be hydrolyzed to be used as wood sugar. Otherwise, the wood cellulose may be formed into cellulose derivatives through various chemical treatments.
A variety of processes are performed to obtain the pulp from the pulp material, which include preparation of the pulp material, pulping, and purifying of the pulp. To easily pulp the wood material, the processes of cutting, barking and sorting are carried out according to the kind of the pulp material. The process of obtaining the fiber from the prepared pulp material is referred to as pulping, which is the most important process in the pulp manufacture.
With the aim of forming the fiber, a composite intercellular layer of the pulpwood is broken using a wood pulp grinder, or is softened using water vapor and then broken by physical force. Pulp obtained through simple mechanical treatment without chemical treatment is called mechanical pulp. Mechanical pulp is advantageous because of a high yield and low manufacturing costs, but unsuitable for use as high quality paper stock because of a high lignin content
Treatment of the pulp material using a chemical for lignin removal results in the composite intercellular layer being dissolved and thus dissociated into a fibrous material. The pulp obtained in such a method is called chemical pulp. Upon manufacturing the chemical pulp, the bulk of lignin of the cell membrane as well as lignin present in the intercellular layer of the pulp material is removed. Simultaneously, large amounts of hemicellulose are dissolved, and a small amount of cellulose is decomposed. Although chemical pulp is high quality, that is, it has highly pure cellulose, it has a lower yield and higher manufacturing costs compared with mechanical pulp. The chemical pulp manufacturing method is exemplified by sulfite pulping, soda pulping, sulfate pulping, etc.
The cleaning process functions to remove non-pulped portion and impurities from the pulped fiber by washing and sorting. Then, as necessary, the bleaching process may be performed. In addition, to obtain high quality rayon pulp, a specific purifying process is carried out.
The above descriptions concern the general pulp manufacturing process using pulpwood. However, according to increasing wood exhaustion over the world, producing paper pulp while protecting the forest and environment is a problem awaiting a solution in the related art. To overcome the problem, techniques of manufacturing paper pulp from non-wood plant fibers mainly by using one- or two-year-old plants have been proposed.
Non-wood plants usable as the pulp material include, for example, bast fiber of paper mulberry, linen, hemp, cotton plants, Manila hemp, rice straw, bagasse, etc. In general, non-wood plants have a large amount of pectin, hemicellulose and inorganic materials and a small amount of lignin. Upon pulping, non-wood plants are subjected to chemical pulping, semi-chemical or mechanochemical pulping, and can be formed into unbleached or bleached pulp under milder conditions, unlike wood.
Non-wood pulp has different properties according to fiber forms, chemical compositions, non-fiber cell types and amounts. Therefore, the paper made using the non-wood pulp alone or in combination with the wood pulp can be easily controlled in terms of strength, durability, electric properties, gloss, dimensional stability and printability, and thus, be used as various applications, with wide use ranges.
However, to manufacture chemical pulp for paper using non-wood plant fiber, a process of soda pulping, sulfite pulping or kraft pulping is mainly adopted. Upon manufacturing pulp, a large amount of a sulfur compound, such as Na2SO3 or Na2S, as a beating agent is used in the sulfite and kraft processes. This compound generates offensive smells and aggravates wastewater. As a sulfur-free pulping method, there is proposed a beating process using soda. However, the use of soda alone results in low pulp yield and low paper strength. To alleviate the problems, the use of anthraquinone along with soda has been proposed, but anthraquinone has difficulties in preparation of the beating agent and in biodegradation thereof. Further, anthraquinone is expensive, thus increasing the manufacturing costs of the non-wood pulp.
In this regard, Korean Patent Laid-open Publication No. 2001-1550 discloses a method of manufacturing pulp using corn as herbaceous plant. By using the corn stem as paper pulp material, it is possible to make paper having high quality like Korean paper, with low manufacturing costs.
However, the above method is disadvantageous because it uses a toxic chemical, thus causing environmental contamination
Japanese Patent Laid-open Publication No. Hei. 3-199486 discloses a method of manufacturing paper and binder fiber using a water-soluble polysaccharide. The usable water-soluble polysaccharide includes agar, carrageenan, alginic acid, etc. The above method is characterized in that an aqueous solution of water-soluble polysaccharide is added to a solvent having hydrophilicity while being poorly soluble to the water-soluble polysaccharide, to obtain a fibrous precipitate. Such a precipitate is applied in the fields of edible packaging for foods and medicines. However, since the film material is obtained by practically using the method as described above, it is impossible for the film material to be of practical use as a paper.
In addition, Korean Patent Laid-open Publication No. 1999-34085 discloses a method of manufacturing a film as a substitute for cellophane, using a carrageenan biopolymer. The invention discloses that carrageenan, which is extracted under mild conditions and has excellent film-forming properties, can be substituted for the plastic cellophane material that generates environmental wastes. However, as a result of actual experiments by the present inventors, the resultant film is very low in strength and cannot be used in practical applications. That is, an additional process using an additive is required.